Process of diminishing of ridging in 17-chrome stainless steel



P 1958 J. H. WAXWEILER 2,851,384

PROCESS OF DIMINISHING 0F RIDGING IN l'I-CHROME STAINLESS STEEL Filed July 3, 1953 2 Sheets-Sheet 1 Hal.

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p 9, 1953 J. H. WAXWEILER 2,851,384

PROCESS OF DIMINISHING 0F RIDGING IN 17-CHROME STAINLESS STEEL Filed July 3, 1953 2 Sheets-Sheet 2 IN VEN TOR. Jane's fl. lflrxwem 5 ATTORNEYS.

United States Patent Ofifice PROCESS OF DllVIINISHlNG OF RIDGING 1N 17-CHROME STAINLESS STEEL Application July 3, 1953, Serial No. 365,984 11 Claims. (Cl. 148-12) My invention relates to the production of stainless steel sheet and strip (hereinafter collectively referred to as sheet stock) of the so-called straight chrome type; and it has for its principal object the diminishing of the phenomenon of ridging in such sheet stock when the stock is drawn.

This and other objects of the invention, which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, I accomplish by that procedure and those teachings of which I shall hereinafter set forth certain exemplary embodiments.

Reference is made to the accompanying drawings wherein:

Figure l is a photographic view of a cup-shaped article drawn from stainless steel and clearly exhibiting the phenomenon of ridging.

Figure 2 is a photograph of four test pieces of stainless steel exhibiting varying degrees of ridging in accordance with a grading system hereinafter described.

Figure 3 is a photograph of two sink strainer baskets drawn from stainless steel, the one to the left being made from a ridging material and the one to the right being made from material produced in accordance with the present invention.

Figure 4 is a chart showing the relationship of ridging to the austenite potential of the material.

It is often desirable to employ draws in the formation of articles from straight chrome steel; and it may be pointed out that in view of the present scarcity of nickel and the unavailability of this metal for the production of consumer goods, many articles which previously were made of 18-8 chromium-nickel steel are now perforce made from straight chrome steels, hereinafter referred to as 17-chrome steels. Seventeen-chrome steels fall under the American Iron and Steel Institute (A. I. S. I.) classification Type 430.

One of the disadvantages of the use of 17-chrome steels is that roping or ridging occurs when the cold rolled and annealed sheet or strip is drawn. The defect appears as alternate ridges and grooves running in the cold rolling direction. Ridging must be sharply distinguished from the phenomenon of stretcher straining. Stretcher strains, or Luders lines (elongated markings sometimes appearing on drawn articles), are a function of yield-point elongation and can be alleviated or eliminated by temper rolling the sheet stock before drawing the articles. Ridging cannot be so eliminated or minimized. Stretcher straining, moreover, occurs in about the first 5% of the elongation due to drawing, and then disappears, while ridging becomes noticeable first at about this point and increases to a maximum just before the failure of the metal in tension.

In Figure 1, which is a photograph of a drawn article, the phenomenon of ridging is clearly apparent on the inside wall. For use in my development and to facilitate an explanation of the results obtained, I have set up a ridging standard, as illustrated in Figure 2, in pieces of metal marked respectively 1, 2, 3 and 4. The samples l atented Sept. 9, 1958 so illustrated were prepared by stretching sheet materials in tension to the point where they begin to neck, i. e. nearly to failure in tension; an increase of tension would have produced failure of the samples. The sample marked 1 has a very slight amount of ridging and is a very desirable material for most purposes. A grade 0 (not illustrated) is exemplified by a material exhibiting no ridging upon similar treatment. Grades 2 and 3, as illustrated, show progressively greater ridging, while grade 4 shows such an amount of ridging as to constitute a severe defect in drawn materials.

It is somewhat difficult toshow photographically the phenomenon of ridging, although it is readily apparent to the eye. In making the photograph which is Figure 2 of this application, the necked portions of the samples were rubbed lightly with a fine emery paper or cloth, resulting in a polishing of the surface; and the samples were illuminated by oblique light. Thus, in the case of the sample marked 1, the very slight amount of ridging present resulted in a polishing of substantially the entire surface of the necked portion, from which the oblique light was not reflected into the lens of the camera. Consequently, the necked area appears dark; but the darkness is fairly uniform. In the case of the other samples, the presence of substantial or considerable ridging resulted in the polishing of the tops of the ridges but not of the valleys between them, so that the darkened area is broken up by vertically extending lighter stripes or streaks. The more extensive the ridging, the better defined these streaks become. Certain diagonal markings in the necked areas of the samples should be disregarded. They relate to an entirely different phenomenon and are caused by stretching the material almost to failure.

It will be understood that ridging impairs the appear ance of the product as drawn, and while satisfactory articles may be made from ridged products, the finishing operations are greatly increased in expense and inconvenience by the necessity for grinding the affected areas to eliminate the appearance of the ridging. It is frequently the practice to buff or polish stamped or drawn stainless steel articles. If ridging is absent or is very slight, such buffing or polishing will ordinarily produce a satisfactory appearance. However, where substantial ridging is .present, bufiing or polishing (whether by mechanical means or by various procedures of electrolytic polishing which have been developed for stainless steel), will not remove the appearance of ridging, and the only way to eliminate the appearance is by subjecting the surface to such a removal of metal by mechanical means as will cut down the ridges to the level of the valleys, an operation which is appropriately called grinding, by whatever means it may be performed.

It does not follow that for all uses the tendency toward ridging must be eliminated from the products employed. It has already been explained that the appearance of ridging is not generally noticeable if the draw involves about 5% elongation or less, and that ridging increases with the percent elongation or depth of draw. As a consequence, many materials, which can be classed as ridging, will be found suitable for certain uses; and in diiferent uses, different degrees of the ridging tendency may be tolerated. Some drawn articles are more easily buffed or ground than others. While my invention is directed in part to the elimination of the ridging tendency in 1 7-chrome steels, it should be understood that its utility is not so limited but has to do with minimizing the tendency, and bettering the grade of such steels insofar as ridging is concerned, so that in many instances a steel may be produced capable of performing satisfactorily in a certain use,vwhereas the most nearly comparable prior art materials could not, even though some tendency toward ridging is still characteristic of the new 3 material. It may be pointed out that the previously available 17-chrome stainless steels of the A. I. S. I. Type 430 analysis have ridging tendencies responding generally to grades 3 and 4 in my classification.

Attempts to alleviate the ridging tendency have hitherto been made. For several years it has been known that a normalizing heat treatment (e. g. heating the material to 1850 F. or thereabouts followed by a rapid cool such as a cooling in air) at an intermediate gauge would considerably reduce the ridging tendency in the final product. By an intermediate gauge, I mean a thickness heavier than the final thickness and existing at some point intermediate cold rolling treatments. Actually difiiculties encountered in further cold reducing the so-treated prodnet, and the open surface of the final sheet or strip stock produced by such normalizing treatment, have forced abandonment of this approach to the problem. It has also been suggested that ridging might be reduced by adding a material such as titanium which has the effect of tying up or fixing carbon or nitrogen, or both. But it has been found that l7-chrome stainless steels containing suflicient titanium to render them completely ferritic even at temperatures of 2000 F. are no better in ridging properties than similar steels not containing the titanium.

Sheet and strip stock of 17-chrome steel is normally made by hot rolling the material to a certain gauge, heat treating it and then cold rolling it to final gauge in one or a series of cold rolling stages. By a cold rolling stage is meant whatever cold rolling reduction is carried on without an intermediate anneal.

Ridging is apparently a phenomenon promoted by preferred orientation and diminished by orientations tending more towar'd the random. Hence, a partial randomization of the grains, as can be accomplished by a phase change or transformation, will have an effect in minimizing ridging.

A high temperature treatment, in which the material is carried to a temperature above the critical followed by a rapid cooling, is of assistance in breaking up or re orienting the original hot rolled grains. The critical temperature for l7-chrome material is around 1600 F. A normalizing treatment may be given the material at the hot rolled gauge, but it will have a greater effect if practiced nearer the final gauge. At the same time the combination of a high temperature plus a pickle serves to open up and toughen the surface of the product, which is undesirable. If the normalizing could be done in a protective atmosphere, the surface problems would be somewhat alleviated, although not entirely solved.

The ridging of l7-chrome stainless steel in some cases may be diminished by cross rolling, since this tends toward a more random and hence a more uniform disposition of the metal components. The cross rolling may occur during hot rolling or during cold rolling, or both. But it will be evident that whatever benefit may be derived from cross rolling as such, it is an available expedient only in the case of materials which can be cross rolled, and is, therefore, not generally available in the continuous production of strip. Furthermore, it is quite difficult to apportion the amount of work done in the two directions of rolling, so that rolling in the second direction will obliterate the effects of rolling in the first direction.

I have found that excellent results without the disadvantages outlined can be attained by controlling the composition of the l7-chrome steel so as to secure a higher amount of austenite at the normalizing temperature. I alter the composition of the steel so that it becomes or approaches a martensitic steel rather than the normally ferritic product, the steel thus becoming an air-hardening steel.

An austenitic condition in the steel at a high temperature can easily be converted to ferrite by a procedure involving slow cooling. On the other hand, a rapid cooling may result in the conversion of some or all of the 4 austenite to martensite, which is a very much harder modi fication. The term air hardening is used in the art to indicate a steel in which a cooling in air from a high temperature will result in the conversion of austenite to martensite in sufficient quantity to produce an appreciable hardening.

In plain carbon drawing steels, the bare fact that one sheet is harder than another does not necessarily imply a radical difference in drawing performance. However, in the materials to which this invention relates, a softer material will draw better than one which has a higher hardness. Since the material is air-hardening, I practice a sub-criticalanneal, as hereinafter set forth, after each normalizing treatment or its equivalent.

The changes in composition and processing hereinafter taught tend to eliminate or greatly minimize the ridging tendencies providing there has been at least one heating to above the critical, as for example, in slabbing or hot rolling. For maximum non-ridging, additional transformation heat treatments can be practiced. I shall hereinafter refer to the analysis of the steel as meaning its composition, and the processing of the steel as meaning the physical operations practiced thereon. By thermal history, I mean the entire history of heat treatments of the metal from the ingot until the final sheet stock is produced. In the practice of my invention, the better the analysis of the steel in accordance with the teachings hereinafter made, the less high temperature processing will be required to secure a particular low ridging grade. Also, the hotter and longer the steel is heated, and the closer to the final gauge the heating is performed, i. e. the better the thermal history of the steel, the less the analysis need be altered from that of the commonly produced or previously available A. I. S. 1. Type 430 to secure a particular low ridging grade. Nevertheless, I have found that the matter of analysis is of very great importance in considering the phenomenon of ridging, to which thermal history is preferably an adjunct, the best possible results being secured in material of the best analysis which has also had the best thermal history, in the light of the teachings to follow.

Considering, first, the matter of analysis, it may be pointed out that the A. I. S. I. Type 430 limits for a l7-chrome steel are:

Percent Cr 14.00-18.00 C, maximum .12 Mn, maximum 1.0 Si, maximum 1.0 P, maximum .040 S, maximum .030

In accordance with my teachings, the ridging tendency will be improved, the greater the amount of austenite which can be formed in the steel, i. e. its austenite potential. I have developed a formula whereby the austenite potential of an A. I. S. 1. Type 430 steel can be predicted and controlled. Reference is made to the austenite potential of the steel because the betterment of the product arises from changes or at least is related to changes taking place at high temperatures, above the critical, where austenite is present. The amount of austenite formed at such temperatures, however, may be measured by reference to the amount of martensite (which is an austenite transformation product) produced upon quenching to room temperature. For metallographic examination for determining the austenite potential, I normally quench samples in water from above the critical temperature rather than cool them in air since the transformation products are easier to measure in water quenched samples. It is to be understood however, that for the commercial amelioration of ridging, an air cool will be practiced in the plant, as hereinafter set forth.

Reference is made to the accompanying chart, Figure 4, wherein the austenite potential in terms of percent is indicated on the abscissa and the ridging grade in accordance with the classification described above is indicated on the ordinate. An area A has been outlined on this chart showing the usual range of austenite and the corresponding ridging performance occurring in the common, previously available A. I. S. I. Type 430 steels using the hitherto usual sub-critical open anneal (at about 1500 F.) at an intermediate gauge and at the final gauge, with the hot rolling having been done above the critical. The Type 430 stainless steels will vary from, at most, 35% down to austenite potential, with the average of all commercial heats being at about austenite potential.

Within the area demarked by the lines B and C on the chart, it will be seen that very substantial improvements in ridging performance can be obtained simply by varying the austenite potential, while still employing the sub-critical open anneal at intermediate gauge and at final gauge. The chart, Figure 4, assumes that the hot rolling has been carried on above the critical temperature.

The values found in the area between lines B and C on the chart are indicative of variations in ridging grade or performance which can be attained merely by varying the analysis factor.

My formula for computing the austenite potential or percentage of austenite is as follows:

Percent A=288 C+350 N+22 Ni+7.5

Mn18.75 Cr-54 Si+338.5

From this equation it is possible to select what I will term an aim spot for a commercial heat, the purpose being that of reducing the ridging tendency while staying within the A. I. S. I. limits for this grade of steel. The austenite potential can be increased in the steel by increasing the quantity of any one or a combination of those elements which have plus values in the formula, or by diminishing the quantity of any one or a combination of those elements which have minus values in the formula, or both. Further, if a particular austenite potential is desired, and it is determined in advance that it is to be secured by a variation in the quantity of some one element or combination of elements, it becomes possible from the formula to calculate what the quantity or quantities of the chosen element or elements should be, the other elements being known and remaining constant.

For example, if a heat were to have Percent Cr 17.00 Si .40 e C .11 Mn .90

it can be calculated from the formula what amount of nickel would be required to give a desired austenite potential providing the quantity of nitrogen were known. 1

Or if the above steel contained a known quantity of nickel, say .25 it can be calculated from the formula that the amount of nitrogen required to produce steel capable of developing 90% austenite would be 135%. Since in making a heat, carbon, manganese and silicon can be held within rather narrow limits by one skilled in the art, one convenient way of practicing the invention is to determine the residual nickel and to insure the presence of enough nitrogen to give the desired austenite potential with the anticipated chromium content.

The nitrogen content of a heat can be controlled, as is understood in the art, through the use of nitrogen-bearing ferro-chromium or nitrided electrolytic manganese. Although nickel is not included in the A. I. S. I. specification. Type 430 is considered to be within limits if it contains less than l-l.25% nickel. Therefore, since the A. I. S. I. definition of Type 430 stainless steel does not specify any nitrogen content either, it will be evident that if the steel contains chromium, carbon, manganese, silicon, phosphorus, and sulphur within the limits of the definition, it will be an A. I. S. 1. Type 430 stainless steel irrespective of its nitrogen content, and irrespective of gredients, my alloys consist essentially its nickel content within reasonable limits. other reason why controlling the autsenite means of the nitrogen or nickel contents or both is feasible where a primary purpose is to stay within the A. I. S. I. specifications for a Type 430 stainless steel.

However, it will be evident from the equation given above that all of the elements set forth therein have their weighted effect in controlling the austenite potential, so that I do not wish to limit myself to any exemplary procedure for controlling the austenite potential. The objects of the invention can be attained by control of the proportions of any one or all of the ingredients, carbon, nitrogen, nickel, manganese, chromium, and silicon. It will be understood that with the exception of these inof iron with such minor but normal impurities as are likely to be found in steels of the A. I. S. I. Type 430 grade, or such additional elements as might be added without materially affecting the austenite potential.

On the accompanying chart, Figure 4, as has been said, the lines B and C roughly demark an area in which very substantial improvement of the ridging behavior of steel can be attained by variations in the steel analysis tending toward increasing the austenite potential. It will be observed that the area in question slants diagonally downward on the chart as the austenite potential increases. Ihe spread between lines B and C along any vertical axis drawn on the chart represents the spread in ridging performance which can be generally expected with the small variations in processing such as amount of cold reduction, annealing temperature, hot rolling temperature, number of intermediate anneals, etc. which occur between heats in standard practice and which might be cumulative. The precise location, therefore, of the lines B and C as lines could vary with control of the specific conditions. The chart is presented in its present form as representing between lines B and C an area containing or more of all heats produced by varying any or all of the processing factors which have been set forth above. It has already been pointed out, in addition, that the positions of the lines B and C on the chart are determined for analysis alone, using what has hitherto been the usual sub-critical open anneal at intermediate gauge and final gauge, the hot rolling having been begun with the metalat a temperature above the critical. The chart demonstrates that, these last mentioned conditions remaining the same, the greater the austenite potential, the better the performance of the material as respects ridging.

The line C can be lowered arrow) in the direction of an proving what I have termed the thermal history of the material. In the manufacture by continuous cold rolling of A. I. S. I. Type 430 stainless steel, the material is cast into ingots which are then converted to slabs and continuously hot rolled to a hot roll gauge which may vary and usually does vary with the final gauge of the material. It has already been indicated that the hot rolling should be conducted at temperatures above the critical to take advantage of the austenite transformation when the analysis is suitable. By this I mean that the slab or rough bar at the start of the continuous hot rolling should be at a temperature above the critical. It is advantageous to finish the hot rolling with the material still above the critical, but it is not always feasible to do this, depending upon the available equipment. A softening heat treatment should be practiced after the hot rolling and before the start of the cold rolling.

Circumstances, including the nature of the metal, and the amount of cold rolling required to carry it to final gauge, will determine whether the cold rolling can be carried on in one part or step, i. e. down to final gauge without an intermediate anneal. It is readily possible in many operations to conduct the cold rollingwithout an intermediate anneal, but as will hereinafter be set forth,

potential by (as is indicated by an area marked D by im- This is anit may frequently be preferred to introduce an intermediate anneal into a routing, even though the metal is capable of beingreduced to final' gauge without it; In the formation of strip by continuous hotand cold roll ing, there will-be a final heat treatmentor series of them. In the production of sheet stock by non-continuous methods, the same general procedure will be carried out, although inthe use of individual hot mills, it will ordinarily be the practice to carry the material down much closer to the final gauge by pack rolling and the like, so that in some instances the cold rolling may be only a skin passing for surface and the like. However, in such procedures the-material will be heated once or a plurality of times before reaching final gauge, after which it will be subjected to another heat treatment.

Thus, in general, there is a possible heat treatment following hot rolling, and a possible one or more heat treatments intermediate-the cold rolling and the final heat treatment; and the thermal history of the material mentioned above embraces-all of these heat treatments which are practiced. All, of them have a certain effect if they are or include a heat treatment above the critical temperature. In general, the specific effect of a heat treatment above the critical temperature will be greater the closer this heat treatment is practiced to the final gauge of the material. Thus, the effect on ridging of a heat treatment above the critical temperature will be greater if that heat treatment occurs intermediate a cold rolling schedule than if it occurs immediately following the hot rolling. But the effect of heat treatments is also cumulative and one can generalize by saying that the thermal history of, the material is improved the greater the number of heat treatments above the critical to which it is subjected. It will be understood by the skilled worker in the art that various routings may be adopted depending upon the equipment available and the desired qualities ofthe finished product, so that it is not practicable to set forth some specific routing as the most desirable since another routing may be preferred or even rendered necessary by special circumstances.

A heat treatment above the critical temperature is preferably carried on forthe sake of economy in a continuous furance, i. e. a heat treatment in which the material is carried through an elongated furnace in the form of a single strand or single layer so that its surfaces areopen to the atmosphere of the furnace, whatever that may be. In such a furnace the material can readily and rapidly be carried to'a high temperature such as a temperature well above'the critical, and can be rapidly cooled either by being brought out into the air or by beingpassed through a cooling hood in a controlled atmosphere. In the practice of my invention, however, I have found that a heat treatment of this kind, which for convenience I shall hereinafter refer to as a normalize, should be followedby a sub-critical anneal, by which I mean a-heat treatment at a temperature below the critical. The reason for this is that with relatively high austenite potentials, a rapid cooling from above the critical, such as a cooling in air, will result in the formation of martensite and an undesirable hardness will develop in the material. This is why normalizing treatments hitherto suggested as an expedient for alleviating ridging have not proved successful.

A normalizing treatment is carried on above the critical, which, in materials of the class to which this invention relates, will generally mean above 1600 F. The sub-critical anneal should be carried on at a temperature below the critical, whatever it maybe, as for example, a temperature around 1500 F. The sub-critical anneal may take various forms, but ordinarily an open anneal at around 1500 F. will serve since such processing will satisfactorily convert all of the martensite to ferrite, and provide the necessary softness in the material where it is intended for d'eep drawing'. A final open anneal at 1500 FJortlier'eabciuts, followed by a temperrolling, will al ways he theminimum: heat treatment required after completion of cold rollingforthe development ofthe' final physical properties. For some uses itmay be desirable to=box anneal the product. A box anneal above 1690 F.

will' serve as aheat treatment above the critical, and the slow cool normally following such a box anneal will serve as the sub-critical anneal mentioned above in producing sufiicient softness. Thus, as a final heat treatment, one may practice either a sub-critical open anneal followed by a temper rolling, or a normalizing plus a subcritical open annealing followed by a temper rolling, or a box anneal with. or without the utilization of a supercritical temperature followed by temper rolling. One may also practice an open anneal at sub-critical temperature ora normalize. followed by a sub-critical open anneal as a preliminary to skin-passing, and then, if desired, subject the material to a box anneal at sub-critical or supercritical temperatures. All of these heat treatments, whether occurring before or after skin passing, I embrace in the term final heat treatments. No final heat treatment or series of them will appreciably affect the ridging characteristics of the steel unless there is included the step of heating the material above the critical. Where the material is heated above the critical in a final heat treatment or series of heat treatments, it should then be subjected either to slow cooling as described, or to a subcritical anneal..

The final heat treatment or treatments need not involve heating the material above the critical if this is done earlier in the routing, as in hot rolling, or as in a heat treatment following hot rolling or a heat treatment intermediate the stages of cold rolling, as set forth above. Subject to'the qualification that the closer the supercritical heat treatment comes to the final gauge of the material, the greater its effect will be, the greatest possible effect of the thermal history of the material on its ridging behaviour will be attained if at. all gauges where heat treatment is practiced, the step of carrying the material above the critical temperature is included.

Improvements in the thermal history of the material serve to lower the line C toward or into the area D, as has been explained, an area being indicated rather than a line because of the variations to be expected in ordinary manufacture within the purview of my teachings. The effects of identical thermal history will vary with different alloys. In my preferred procedure, I stay within the ranges of carbon, manganese, silicon, phosphorus, sulfur and chromium which are comprised in the A. I. S. I. definition of Type 430 chromium steel. When I do this, the mechanical properties of my non-ridging material are similar to those of the known l7-chrome steels of the specification, and the metallographic structure of my new material, as determined by ordinary methods, is also similar to that of the commonly available A. I. S. 1. Type 430 materials, which is notoriously susceptible to ridging. While I am not limited to steels falling within the A. I. S; I. Type 430 definition, steels departing radically therefrom, as by the inclusion of very large amounts of those ingredients which raise the austenite potential in accordance with the formula given above, may exhibit differences in mechanical properties and metallographic structure which may or may not be desirable for any specific use. However, from the standpoint of analysis, the greater the austenite potential of the steel, the more free it will be from ridging characteristics.

It may be pointed out that where titanium is added to l7-chrome steels to tie up carbides, and to prevent formation of austenite at high temperatures, the austenite potential will be exceedingly low or zero, and to attempt to raise the austenite potential would defeat the purpose of the titanium addition. In such steels only the thermal history can be depended upon to make what little im provement in ridging performanceis possible.

It should; be. emphasized that: the. utility of my inven:

tion is not limited to materials or processes of making them in which the ridging characteristics are done away with entirely. It is of great value in improving the ridging performance of chromium steels and for many uses the difference of a grade or less is sufiicient to distinguish a satisfactory from an unsatisfactory product as to ridging when the end use is considered. While it is true that for most uses, if in accordance with my teachings the austenite potential of the material is carried to around 60% or higher as shown on the attached chart, the material to all intents and purposes will be free of ridging, the A. I. S. I. Type 430 steels hitherto available have not had more than about 35% austenite potential, averaging very much lower, and hence have ranged from about 2.5 to 4.0 on my grading scale. The teachings of this application may be used without significant increase in expense to increase the austenite potential to values above about 35% and higher, thereby producing marked and valuable betterment in the ridging performance of such steels.

Modifications may be made in my invention without departing from the spirit of it. Having thus described my invention in certain exemplary embodiments, what I claim as new and desire to secure by Letters Patent is:

1, A process of diminishing ridging upon drawing in ferritic chromium-bearing steels having a chromium content of substantially 14.00% to 18.00% chromium with less than about 1.25 nickel Which comprises increasing the austenite potential of said steel to a value in excess of substantially 35 and subjecting the steel to a heat treatment above the transformation temperature followed by a heat treatment therebelow to convert the formed martensite to ferrite.

2. The process claimed in claim 1, in which the heat treatment above the critical temperature is a normalizing heat treatment followed by a rapid cooling, and in which the heat treatment below the critical temperature is an open anneal at a temperature of the order of 1500 F.

3. The process claimed in claim 1, in which the heat treatments above and below the critical temperature constitute parts of a box annealing procedure.

4. A process of reducing the ridging tendencies of ferritic 17-chrome stainless steel which comprises producing a steel in accordance with the following specification:

Percent C, maximum .12 Mn, maximum 1.0 Si, maximum 1.0 P, maximum .040 S, maximum .030 Cr 14.00-18.00

with variable amounts of Ni and N, balance substantially all iron except for normal impurities, the Ni being less than about 1.25 and controlling the austenite potential of said steel in accordance with the formula:

Percent A=288 C+350 N+22 Ni+7.5 Mn- 18.75 Cr54 Si+338.5

to a value in excess of substantially 35 reducing said steel to sheet gauge, and subjecting said steel to a heat treatment during the processing at above the critical temperature.

5. A process of reducing the ridging tendencies of ferritic 17-chrome stainless steel which comprises producing a steel in accordance with the following specification:

Percent C, maximum .12 Mn, maximum 1.0 Si, maximum 1.0 P, maximum .040 8, maximum .030 Cr 14.00-18.00

'10 with variable amounts of Ni and N, balance substantially all iron except for normal impurities, the Ni being less than about 1.25%, and controlling the austenite poten tial of said steel in accordance with the formula:

Percent A=288 C+350 N+22 Ni+7.5 Mn- 18.75 Cr-54 Si+338.5

Percent A=288 C+350 N+22 Ni+7.5 Mn- 18.75 Cr-54 Si+338.5

to a value in excess of substantially 35 and reducing the steel to sheet gauge by hot rolling followed by cold rolling, the said process being characterized by hot rolling it above the critical temperature and an open anneal at substantially 1500 F. following cold reduction to gauge.

8. A process of diminishing the ridging characteristics of ferritic stainless steel containing substantially 14% to 18% chromium and less than about 1.25% nickel which comprises controlling the austenite potential thereof in accordance with the formula:

Percent A=288 C+350 N+22 Ni+7.5 Mn- 18.75 Cr54 Si+338.5

to a value in excess of substantially 35%, and reducing the steel to sheet gauge by hot rolling followed by cold rolling, the said process being characterized by hot rolling it above the critical temperature and an open anneal at substantially 1500 F. following cold reduction to gauge, and by at least one heat treatment in which the temperature of the steel is carried above the critical temperature, followed by an anneal at a temperature therebelow.

9. The process claimed in claim 8, wherein the last mentioned heat treatments comprise a normalizing followed by an open anneal at substantially 1500 F.

10. A stainless steel of improved ridging characteristics produced in accordance with the process of claim 3.

11. A stainless steel of improved ridging characteristics produced in accordance with the process of claim 4.

References Cited in the file of this patent UNITED STATES PATENTS 1,337,209 Eilender Apr. 20, 1920 1,337,210 Eilender Apr. 20, 1920 2,118,693 Arness May 24, 1928 2,384,567 Schaufus Sept. 11, 1945 2,451,469 Brophy et a1. Oct. 19, 1948 2,673,166 Carruthers Mar. 23, 1954 OTHER REFERENCES 

8. A PROCESS FOR DIMINISHING THE RIDGING CHARACTERISTICS OF FERRITIC STAINLESS STEEL CONTAINING SUBSTANTIALLY 14% TO 18% CHROMIUM AND LESS THAN ABOUT 1.25% NICKEL WHICH COMPRISES CONTROLLING THE AUSTENITE POTENTIAL THEREOF IN ACCORDANCE WITH THE FORMULA: 